Hello. I need some advice. My 1971 Olds 98 is having brake troubles. It started a number of weeks ago when the brake warning light came on occasionally. Whenever the brake light came on the brakes would fade and I would have to correct the problem by tapping on the brake pedal. I assume this was being caused by the proportioning valve.

After this happened a few times the brakes failed completely. Luckily I was in the driveway. I stopped the car in the yard. The car sat for a week or two. This morning I visually inspected all of the brakes. The pads and shoes are in good condition. There doesn't seem to be a catastrophic leak at either of the rear wheel cylinders or the front cailpers.

I bled the brakes with my wife's help. The rears were done easily. When we did the front I had to hold the button down on the proportioning valve while my wife pumped up the brakes. Once she pumped up the brakes and held the brake pedal to the floor I would release the button on the proportioning valve and bleed the caliper. I had good fluid flow from all 4 corners.

I started the car and drove back and forth in the driveway. The bleeding helped; however, the brakes are still malfunctioning. I had to stop the car a few times by dropping it into reverse. The brake light is on occasionally again. I can tap the brakes a number of times to get the brake light to go off.

What is my problem? Do I need to replace the proportioning valve? Would anything else cause this?

If it is the proportioning valve then how do I find one? I did some research on our site and it seems they are hard to come by. I called around at the parts stores and the only one that had anything was Car Quest. They said it was $9.00... I couldn't even find it one Rock Auto or Fusicks. Is this something I can replace with an aftermarket part? If so, I would. I don't care about originality. I just want the car to function properly.

Please help!

 

I don't know what is causing your problem, but I believe that Inline Tube is now selling proportioning valves. Not sure if for full size cars.

 

Have you been loosing fluid? Did you check the master cylinder yet? I had a master go bad and the symptom was I'd step on the brake pedal and it would be firm. But it would slowly go to the floor and then the dash light would come on. I don't know anything about the proportioning valves. John

 

If there are no leaks I would check out the master cylinder too.
If it has brake pressure it should still have brakes either on the front or back even if the combinaion valve is sticking. Assuming it has the distribution block proportioning/hold off valve combo? The light goes on when there is unequal pressure in the system either from a leak in one half of the system or a bad MC is all I could figure. If the pedal gets like a brick is under it check out the booster and the vac lines.

 

I think you have an internal leak in the master cyl....no loss of fluid but the fluid is seeping past the plunger causing the no brake condition. I had a car do this....hit the pedal hard I had brakes. If you hit it soft or gradual pressure it would go to the floor...scarey!

 

Sounds like the master cylinder going bad.

 

In more than 35 years of driving these cars, I've NEVER had a proportioning valve go bad, so I'm curious as to why so many people think this is the problem with their brakes. Start with the rest of the system. The master cylinder is the most likely candidate. It sounds like an internal leak. Reduced pressure in one circuit of the brakes is causing the differential pressure valve to illuminate the BRAKE light. Depending on what has been done to the brake system and when, now may be a good time to replace all the soft parts (hoses, caliper seals, wheel cylinder seals). Did you peel back the boots on the calipers and wheel cylinders to look for leaks?

 

Thanks for the input gents. I did add some fluid to the MC when this started. I never noticed brake fluid under the car either. I will check the MC. I've heard the seals can go bad and they leak into the vacuum booster. Should I check for fluid inside the vacuum booster, or is there a bench test I can do after removing the master cylinder to see if it is bad?

 

weird that you have this problem too, my 71 Olds 88 just got this same problem, i replace both front calipers because i couldnt get the bleeder valves open and they broke inside, I also replaced the MC, thinking that was the problem, bleed the MC on the car, then bleed all the brakes and when the car is off and you pump them the brake pedal gets hard, when you start it though, and push the brake pedal down more than half way the light comes on, and the pedal looses pressure, also my wheel cylinders in my drums are new, i just replaced them not that long ago. My symtoms are just like Olds 64, although i got a new Bendix MC, is the new MC bad or what? thanks

 

I also think it might be the master. I had a similar issue with my 72. no external leaks, low pedal, lousy pressure out of the rear bleeders but the fluid was going somewhere.

 

That is interesting Gustav. I found a master cylinder/vacuum booster at Advanced Auto. I will install it this weekend. I really hope the MC fixes the problem.

 

found out what my problem was today. I had someone press the brake with the car on, and i looked for leaks, i found it still squirting out of the brake hose bolt that goes to the caliper. I already had that thing tight, i tighten it some more and the leak stopped, and lights out. I thought i might strip it out going over kill though. It wasnt leaking on the ground it was just spraying on the shield

 

Good news Gustav. I should have my wife do the same thing while I look under the car for leaks.

 

I would because i checked the brakes when the car was off at first, the brakes got hard and i didnt see any leaks, the minute i cranked it over and someone press the pedal down i seen it, hopefully yours is just a MC or leak something like mine

 

That is a good idea although there would be evidence of fluid on the tires/wheels/floor. I think I recall that you said there was none when we talked earlier this week. I vote for the master cylinder as well. Since you have already purchased one you might as well....

Let us know what happens.

 

yeah the fluid was kind of on the tires and the splash shield but didnt ever hit ground, its better now

 

My Olds is back on the road! It turned out it was the master cylinder. I replaced the master cylinder and vacuum booster. The vacuum booster was a PITA to get in, but it stops like a champ again!

 

Yeah!!!